In order to reap the requirements for size of the devices and switching times in microelectronics, the method for producing interconnection lines, said lines requiring insulation by a porous dielectric material, has had to be adapted. Furthermore, producing narrow trenches in the porous dielectric material, SiOCH for example, requires reviewing the various plasma methods (etching, post-etching treatments) and integration schemes, since this material is known to be easily degraded when it is exposed to a plasma based on fluorine in particular. The studies carried out to evaluate the efficacy of various plasmas according to the materials exposed to the plasmas make it possible to characterise and optimise the methods for transferring patterns of a metallic or organic mask in an SiOCH that is porous or hybrid (made porous at the end of integration). One major problem is the modification of porous and hybrid dielectric materials during “post-etching” plasma. With a fluorocarbon plasma, the hybrid material exhibits etching mechanisms similar to those of a dense SiOCH. The material of the titanium nitride (TiN) type and the organic material have etching mechanisms different from those of dielectrics, which ensures good selectivity. The optimised etching method for the organic mask allows the etching of very narrow trenches with an almost vertical profile.
During the method for producing interconnection lines, one of the critical steps remains the etching of the layer of porous dielectric material. The main drawback of a plasma etching based on fluorine in a device comprising a metal hard mask and a layer of porous dielectric material is the generation of residues resulting from the interaction between the chemical species present in the plasma and on the metal mask; said residues are deposited on the layers and consequently cause failures of the devices produced. This phenomenon of the formation of residues during plasma etching based on fluorine and carbon is in particular described in the publication in English: “Residue growth on metallic-hard mask after dielectric etching in fluorocarbon-based plasmas. I. Mechanisms”, N. Posseme et al, J. Vac. Sci. Technol. B 28(4), July/August 2010, pp. 809-816.
Other drawbacks of plasma etching based on fluorine are mentioned below and illustrated in FIGS. 1a to 1c. These figures depict in particular the various problems caused by the etching of the porous dielectric, p-SiOCH, using a fluorocarbon plasma and by the use of a hard mask of titanium nitride (TiN).
FIG. 1a illustrates the structure of the interconnection lines after etching in a fluorocarbon plasma of the porous material used as a dielectric material, p-SiOCH, and before filling of the trenches 110, as implemented in the conventional so-called “damascene” method of formation of the copper interconnections.
The etching is done using a hard mask 120, typically made from titanium nitride (TiN), which covers a layer 130 of silicon oxide (SiO2) serving as a stop layer during the chemical mechanical polishing (CMP) step with a view to the subsequent production of interconnection lines based on copper, for example with the “damascene” method. Prior to the depositing of the layer 130 of silicon oxide, a layer of porous dielectric material 140 is formed, for example SiOCH. Under the layer of porous dielectric material 140 is a layer 150 serving as a stop layer during the plasma etching. The global layer 160 represents all the underlying layers of the integrated circuit, in particular those containing active components that are produced during the preliminary steps of the methods, those that precede the so-called “BEOL” (back-end of line) operations during which, at relatively low temperatures, all the levels of interconnections between the active components are produced and where all the dielectric layers will be etched successively.
FIG. 1a illustrates a first problem related to the greater sensitivity of the porous dielectric material 140 during the conventional etching in a fluorocarbon plasma. A greater consumption of the porous dielectric material 140 is observed, corresponding to an over-etching of this layer 140. Moreover, problems of absorption of moisture are also encountered, which could give rise to an increase in the dielectric constant of the low-permittivity dielectric material; which runs counter to the aim sought and also affects the reliability of the device produced.
FIG. 1b illustrates an additional problem that relates to the use of a metal hard mask 120. After etching, the trenches 110 are inevitably exposed to free air, causing a deposition and a formation of residues 170 on the walls of the trenches 110. The formation of these residues 170 is all the greater, the to longer the trenches 110 are left in free air. The mechanism of formation of the residues 170 is associated with the fluorinated species present on the hard metal hard mask 120 and on the surfaces of the low-permittivity porous dielectric material 140, after etching in the fluorocarbon plasma. The fluorinated species then react with the moisture in the air, producing hydrofluoric acid (HF), which forms metal salts in contact with the hard mask 120. These residues 170 affect the quality of the following depositions: that of the layer forming a barrier to the diffusion of copper, and that of the layer of copper that will serve to form all the metal interconnection lines (the use of copper requires the prior deposition of a layer intended to prevent the diffusion of it in the semiconductor material used for producing transistors, generally made from silicon). These residues 170 may produce unwanted vias between lines (for example short-circuits) and line ruptures (for example open circuits) that very significantly affect the manufacturing yield of the devices produced.
FIG. 1c illustrates yet another problem that is related to wet cleaning based on the use of hydrofluoric acid (HF) and which proves not to be sufficiently effective to eliminate all the fluorocarbon layer formed during etching. After wet cleaning, fluorine remains on the flanks of the trenches 110. The fluorine is encapsulated during the deposition of the layer 195 serving as a metal barrier and which must be deposited before the deposition of copper to prevent the migration of the latter. The fluorine has a tendency to diffuse and may lead to a degradation in reliability of the porous dielectric material 140, for example a p-SiOCH. An aggressive wet cleaning, which would remove all of the fluorine layer, would have the drawback of leading to a loss of definition of the critical dimensions of the devices produced.
Consequently, introducing low-permittivity porous dielectric materials necessary for reducing the time constant of the interconnections between active components of an integrated circuit, and in particular etching these porous dielectric materials in a fluorocarbon plasma, pose numerous problems.
One subject matter of the present invention proposes a method for producing interconnection lines limiting, or even eliminating, at least some of the problems and drawbacks mentioned above, and in particular the problems generated during the step of etching porous dielectric materials using a plasma based on fluorocarbon compounds.